The secretion of many toxic, xenobiotic, or pharmaceutical compounds from the body is a major function of the kidney. Renal secretion of these substances involves the integrated activity of a suite of transport proteins in the basolateral (peritubular) and apical (luminal) membranes of proximal tubules. Renal epithelial cells are often a target for xenobiotic compounds, producing nephrotoxicity, because of the central role that proximal tubules play in secretion. Certain types of cancer cells overexpress polyspecific transport proteins, which increase the efflux of a wide range of chemotherapeutic agents from the cell. This confers multidrug resistance, a significant clinical problem, to these cancer cells because transport activity keeps the cytoplasmic concentrations of the chemotherapeutic agents low. Transporters mediating secretion can be classified according to their substrates, handling either organic anions (OAs) or organic cations (OCs). Historically, most attention has focused on renal secretion processes handling of monovalent, low molecular weight (<400 Da), OAs (such as para-aminohippurate, PAH) and a cellular model for OA secretion has been widely accepted for several years. The transporter mediating PAN secretion has been cloned and termed OAT, organic anion transporter. Renal pathways for the secretion of OCs (such as tetraethylammonium, TEA) have also received considerable attention. Transporter proteins handling organic cation flux across the basolateral membrane of renal cells have been cloned and termed OCTs. Recent experimental evidence strongly suggests that a transport pathway exists in the kidney which mediates the secretion of larger (>500 Da), polyvalent, conjugated OAs (such as leukotriene C4, LTC4). The transporters mediated the efflux of these substrates across the apical membrane are termed MRPs, multidrug resistance protein. The MRP isoform expressed in the kidney, and also in liver hepatocytes, is called MRP2 (also called cMOAT). Although both MRP1 and MRP2 overexpression have been associated with multidrug resistance, the physiological role of MPR2 in the kidney is undefined. This proposal will focus on transporters handling large polyvalent OAs (MRP2) and organic cations (OCT1) because there is much less known about the physiological role of these transporters and because of important technical issues. In this proposal experiments are outlined that will establish the relationship between long-term and short-term exposure of proximal tubule cells to MRP2 substrates and OCT substrates in terms of transport activity, transporter protein expression, and transporter mRNA expression. We suggest that transport of LTC4 and TEA will be upregulated in renal cells following extracellular exposure to these substrates. Upregulation of transport is expected to be a result of either transporter activity or transporter abundance. It is also expected that changes in transport will be reflected by parallel changes in mRNA expression. These studies will make use of cultured cell models and intact tubules to understand the characteristics of transporters working within the environment of an intact tubule epithelium. The proposed studies should result in a new, general model of the cellular strategy for increasing secretion in response to long- and short-term exposure to xenobiotic anions, and a more specific understanding of the physiological role of these transporters. The latter studies offer the potential of providing insight into intervention sites to alleviate multidrug resistance.